1. Field of the Invention
The invention concerns a bundle of heat exchanger tubes connected downstream of a thermal cracking installation with a bundle of heat exchanger tubes held between two tube plates, a method to coat a tube plate as well as the application of a packing material.
Such bundles of heat exchanger tubes are used, for example, in ethylene plants for the production of ethylene by thermal cracking on the upstream side of a transfer line of a cracking oven and described as cracked gas coolers.
2. Description of the Background
Cracked gas coolers must satisfy extraordinarily high requirements with regard to the construction and material properties. The hot reaction mixture (approx. 850.degree. C.) emerging from the cracking oven during the pyrolysis of hydrocarbons like naphtha, heavy petrol or even ethane has to be cooled quickly in the cracked gas coolers to prevent undesirable secondary reactions. The cracked gas coolers or the bundle of heat exchanger tubes serve as an evaporating boiler, in which high-pressure steam is produced by evaporating feed water supplied from the casing.
The cracked gas arriving at great speed from the cracking oven usually enters in the cracked gas cooler from below via a bonnet socket in which the transfer line is axially arranged and impacts on the bottom tube plate and after passing through the heat exchanger tubes of the cracked gas cooler is conveyed to an oil wash and further processing.
Despite the short residence times and high speeds of approx. 300 m/s the cracked gas already contains coke particles, which have a strong eroding affect at these speeds. As far as the construction of the apparatus is concerned, it is not practically feasible to charge evenly all internal tubes of the cracked gas cooler. consequently, the tubes provided in the central regions of the base plate as well as in the core zone region will erode more than those in the peripheral regions.
From EP-A-0 567 674 heat exchangers for the cooling a synthesis gas generated in a coal gasification plant is known, wherein the tube plate on the side of the gas inlet comprises individual parallelepiped-shaped nozzles, provided next to each other and abutting on the outer edges, while each nozzle has a tapered orifice, narrowing to a tubular cross-section protruding into a heat exchanger tube. This solution does not provide a gas-tight seal between the individual parallelepiped-shaped elements. In the cracked gas coolers of an olefin plant this would lead to coke formation in the intermediate spaces and destroy the materials. Furthermore, the ends of the nozzles used form a tear-off edge in the tube, which in the case of the flow velocities of approx. 300 m/s, used in cracked gas coolers, would cause strong whirling resulting in additional erosions.
Furthermore it is known to provide cooling tubes, installed in a reactor, with an erosion-retarding refractory coating (cf. U.S. Pat. No. 4 124 068), for the purposes of reducing the risk of failure of the tube and the ingress on cooling water into the surrounding. reaction mixture at elevated temperatures.
An attempt has been made to encounter the problem of a considerably stronger flow and load occurring in the core zone when compared with the edge zones by, inter alia, tapered baffles (cf. U.S. Pat. No. 35 52 487) or by diffusor-like deflection devices without baffles (German patent 21 60 372) in the bonnet socket.
Furthermore, to even out the flow-through the bonnet socket on the entry side and also to protect the tube plate from erosion it has been suggested to provide the bundle of heat exchanger tubes with baffles made of bars, bent into rings, wherein the rings are provided along the surface of a taper, the tip of which is directed toward the gas inlet (cf. EP 0 377 089 A1.).
This should slow down the coke particles carried away in the region of the core flow by the gas flowing at high velocity and deflect them partly radially outwards, so that they will no longer result in erosion damages on the tube plate or in the tube. On the other hand such baffles result in an undesired pressure difference and loss of yield due to the corresponding increase of the residence tile.